An apparatus for enciphering data by which enciphered digital information data with improved cipher strength are obtained by subjecting digital information data to enciphered process responding to random number data or pseudo-random number data produced in accordance with key data, and which comprises an enciphering portion for subjecting an HD signal to enciphering process to produce an enciphered HD signal, a cipher producing portion for producing, in response to key data, a cipher data from which random number data or pseudo-random number data are obtained to be supplied to the enciphering portion, a register for supplying the cipher producing portion with input data, and a line number data extracting portion for extracting line number data from the HD signal to be supplied to the register as initial data.